The present invention relates to a construction or a device for cooling a power transistor having a plastic case, wherein the waste heat produced is directed away mainly via the collector lead, the collector load being connected to a collector strip on the circuit board.
Nowadays the RF power transistors used are rather commonly transistors having a plastic encapsulation, often having such a structure that the ends of the collector lead, base lead and often two emitter leads, by means of which the semiconductor piece itself can be connected to the surrounding circuitry, are inside the cast plastic encapsulation. The collector lead is a flat strip considerably wider than the others, and the semiconductor piece itself is on top of this strip. Inside the encapsulation there are connecting leads from the transistor chip to the base and emitter leads. The collector lead may be a few millimeters wide, and owing to the structure presented above it functions as the principle means of transferring the waste heat produced in the transistor into the surroundings. The transistor can be connected to the circuit board by welding the above-mentioned connecting leads directly to the circuit board. Such transistors are used when rather low power levels are involved, in which encapsulation the power loss may be in the order of a couple of watts.
The cooling of transistors such as this is, of course, based on cooling the collector lead of the transistor in a suitable manner. The easiest, although not the most effective, way is to weld the collector lead of the transistor directly to the collector strip on the circuit board, in which case heat will transfer from the collector to the strip on the circuit board and via it further through the circuit board to the continuous metal foil serving as the ground, from which it is transferred into air. This method is not especially effective, because the insulation material of the circuit board itself also serves as a heat transferer, and this material is selected primarily considering its electric properties and not paying attention to its heat transfer properties. It would, of course, be possible to use as the circuit board substrate, instead of the conventional glass-fiber-epoxy material, a suitable ceramic material having good heat transfer properties, but such material is expensive and making in it the various holes needed in the coupling of the components is difficult owing to the hardness of the material.
Another known method is to use a special mounting seat for transistors, fixed to the circuit board for example by means of screws. This seat may be a specially designed metal plate with a large surface area, in the middle of which the transistor is placed. On the surface of the plate there are located electrically insulating but highly heat conductive ceramic pieces over which the collector lead and base lead run, without touching the material of the seat, to the conductor strips of the circuit board. In common-emitter connection the emitter can be connected directly to the seat plate. From the collector lead of the transistor the heat is transferred via a ceramic piece to the seat plate which, having a large surface area, effectively transfers heat to the surroundings both directly and by conducting part of it through the circuit board substrate to the metal foil serving as the ground. This known method is, however, rather complicated and in practice difficult to implement.
One further method of improving cooling is depicted in Finnish Patent Application FI-890791, Nokia-Mobira Oy. In the method described, a thick metal strip is used which is welded on top of the collector strip on the circuit board, the collector lead of the transistor being also welded to the strip. The lead may be in end-to-end contact with the thick metal strip. The waste heat is partly conducted directly from the metal strip into air and partly through the circuit board to the ground foil on the opposite side and from there further into air. The transfer of heat can be enhanced by fixing to the surface of the ground various elements which promote cooling. This method is rather effective and is, furthermore, simple to implement.
It can be stated in general that in a number of practical RF circuitries it is advantageous if a major portion of the waste heat is conducted through the circuit board from the component side to the ground side and from there either directly or, enhanced by additional elements, into air. This is the case especially when the mechanical structure of the device is such that the RF circuit part is on the component side entirely shielded by an RF-tight cover and a continuous ground foil serves as a shield on the other side. In this case the RF shield cover weakens the transfer of heat into air, and so the transfer must in the main be through the circuit board substrate. In such cases the prior-art heat transfer systems described above are usable. However, there are mechanical devices in which it is necessary to cause as large a proportion as possible of the waste heat produced by a low-power transistor to be conducted into air on that side of the circuit board where this component is. The construction according to the present invention provides a system for a need such as this.